Summary of the work packages perfomed by AWI:
(1) Field work on Nioghalvfjerdsfjorden glacier:
The AWI glaciology group joined the Danish glaciologists working on the
floating tongue of Nioghalvfjerdsfjorden Glacier during the summers of
1997 and 1998. The emphasis was on (i) seismic measurements to derive
ice thickness and water depth below the ice shelf, (ii) tilt meter
measurements at the margin, (iii) CTD profiling underneath the glacier
and the sea ice, and (iv) GPS measurements. The measurements revealed a
strong ice thickness gradient near to the grounding line, the existence of
a large cavity below the ice shelf , and the existence of a thick fresh
water layer near to the ice front. These observations support the large
basal melting rates below the ice shelf revealed by the Danish field work.
It is postulated that relatively warm oceanic water which enters the cavity
through Dijmpna Sund provides the energy for melting.
(2) Observations on Storstroemmen glacier:
Stakes planted on Storstroemmen in 1995 were revisited at the end of the
summer season of 1997. Only three stakes were still standing.
Velocity determinations revealed that the overall flow pattern is
still governed by post-surge behaviour.
(3) Flowline model studies:
A flowline model was developed for the lower section of the
Nioghalvfjerdsfjorden flowline. This line was embedded in a 3-D model
of the entire Greenland ice sheet which provided the boundary conditions
at the upper end of the section. Input parameters were provided by the
recent field data collected on the glacier. First results showed that
the flowline model is able to produce quite realistic results and that
the coupling did not introduce unwanted numerical effects or
discontinuities. The meltrates required below the ice shelf to fit
the modelled geometry to the seismic measurements match very well with
the field observations.
(4) 3-D model studies:
A 3-D thermomechanical ice-sheet model was used to refine estimates of the past, present, and future contribution of the Greenland ice sheet to global sea-level changes. Datasets for ice thickness, bedrock elevation, surface elevation, and precipitation rate were first updated to take into account the wealth of new data that became available during the last decade. Also the mass-balance treatment was refined and recalibrated to obtain a best fit to available mass-balance observations. New calculations were performed over the last two glacial cycles to obtain the current evolution of the ice sheet. It was found that the ice sheet is presently close to a stationary state, although that large spatial variations occurred. Patterns and scenarios of future climatic change were downscaled from output of the Hamburg (ECHAM) climate model for the period 1985-2084. A major result was that the total sea-level rise from the Greenland ice sheet (around 5 cm after 100 years) would be substantially less than obtained from earlier model studies. That is due to the small warming predicted for the ice-sheet margin in southern Greenland, where it matters most for melting. An extensive sensitivity study highlighted the role of ice dynamics and the height/ mass balance feedback on the future behaviour of the ice sheet. It was found that ice dynamics cannot be neglected, not even on a century time scale, and that it produces a counteracting effect. Even when greenhouse gas concentrations would stabilize by the early 22nd century, Greenland melt-down is found to be irreversible for equivalent CO2 concentrations more than twice the present value, which would produce a 7 m sea-level rise after a few thousand years.